The invention relates to a method of preparing an ink which can be used for the manufacture of a functional layer, in particular for the manufacture of an electrode for fuel cells, which can be operated at medium or high temperatures. The invention also relates to such an ink, a use of this ink and to a high temperature fuel cell.
In a high temperature fuel cell (SOFC fuel cell), oxidizing gases, on the one hand, and reducing gases, on the other hand, react to electrochemically active elements while producing electrical and thermal energy. With planar fuel cells, the electrochemically active elements are formed in a film-like manner and each include the following functional layers: two electrode layers, the anode and the cathode, on which electrochemical electrode reactions take place between the gases and catalytically active substances of the electrodes, and a solid electrolyte layer which separates the electrode layers and which is a layer of an ion-conductive solid material electrolyte. In simpler embodiments of the electrochemically active element, the solid electrolyte layer forms a carrier structure for the electrode layers. The manufacture of such electrode layers is described, for example, in EP-A 0 902 493.
A high temperature fuel cell is known from DE-A-19 819 453 in which an anode substrate forms a carrier structure. An intermediate anode layer, a preferably very thin solid electrolyte layer and the layer-like cathode are applied to this carrier structure. The anode substrate and the intermediate anode layer are functional layers which are manufactured from the same electrode material, namely from a porous cermet which consists of a ceramic material YSZ (yttrium stabilized zirconium oxide) and nickel.
YSZ is also used for the solid electrolyte structure (carrier structure or layer applied by coating). In a zone at the interface between the anode and the electrolyte, the electrochemical reactions take place at so-called three-phase points (nickel/electrolyte/pore). With these electrochemical reactions, nickel atoms are oxidized by oxygen ions (O2−) of the electrolyte and these are again reduced by a gaseous fuel (H2, CO), with H2O and CO2 being formed and the electrons released in the oxidization being conducted further by the anode substrate. The nickel oxide, with which the redox processes of alternating reduction and oxidation take place, is termed an “active oxide”.
The cathode is also a functional layer in which two solid phases form three-phase points together with a pore space. Oxygen molecules from the pore space react, while taking up electrons, to form oxygen ions which are passed on by the one of the solid phases to the solid electrolyte layer. The electrons are supplied to the cathode from an inter-connector which connects the electrochemically active elements of adjacent fuel cells.
The functional layers can be manufactured with an ink-like material which is applied by means of a screen printing process to a substrate, for example onto the electrolyte layer. After a drying step, the coating material is sintered onto the substrate by firing.
In order for a large turnover of the electrode reactions to result on the electrodes, these functional layers must have a large density at three-phase points. To obtain a large density, the two solid phases must be joined together in the form of very fine particles. It is possible to manufacture such particles by means of chemical processes. However, it has been found that with a manufacture of an ink, these particles cannot be mixed homogeneously. As a result of cohesion forces, homogeneous aggregations form which each consist of a large number of particles of the same solid phase. The required three-phase points only result at the borders between the aggregations.